[Bug middle-end/34678] Optimization generates incorrect code with -frounding-math option (#pragma STDC FENV_ACCESS not implemented)

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=34678

--- Comment #37 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #36)
> Created attachment 46396 [details]
> poor mans solution^Whack
> 
> So this is what a hack looks like, basically sprinkling those asm()s
> throughout the code automatically.
> 
> Note I need to protect inputs, not outputs, otherwise the last
> testcase isn't fixed.

Actually, you need to protect both inputs *and* outputs...

> Improving this poor-mans solution by writing in some flow-sensitivity
> like tracking which values are already protected

At least if you use "=x" (or whatever the right constraint is on each target)
it doesn't really hurt to have a dozen protections on the same variable.

> and if there's a possibly
> harmful FENV access in between maybe in a similar way tree-complex.c tracks
> complex components might work.
> 
> Note that the FENV pragma does _not_ enable -frounding-math (it really has
> no effect!) so you need to supply -frounding-math yourself (or fix the
> frontends to do that).

If you protect even constants, the current effects of -frounding-math become
redundant.